Serial serum specimens from untreated patients with acute schistosomiasis have provided direct evidence that chronic antigenic stimulation is required for IgG4 responses to develop in humans. Despite intense clinical and immunological stimulation, patients were unable to mount specific IgG4 responses for at least 4-9 months after becoming infected; the rapidity of the onset of such responses was directly proportional to the intensity of infection. The kinetics of IgE and IgG4 responses appear similar and distinct from those of the other IgG subclasses. The tropical pulmonary eosinophilia syndrome (TPE) is generally caused by filarial parasites, but recent evidence indicates that strongyloides infection may also induce a syndrome clinically indistinguishable from TPE. Detailed immunologic analysis is underway to define serodiagnostic means to differentiate between these two clinical syndromes.